The invention relates to a method for producing a particular air/fuel mixture in the combustion chamber of an internal combustion engine. Such engine has direct fuel injection, an air inlet which induces an inclined swirling motion to the air inducted into the combustion chamber, a fuel injector arranged centrally on the upper side of the combustion chamber for injecting a fuel spray into an injection volume, and a spark plug which is arranged centrally on the upper side of the combustion chamber and the electrodes of which are situated outside the injection jets of the fuel injector.
A gasoline, four-stroke engine having direct fuel injection, in which stratified lean operation is possible, is known from the prior art. With operation of this type, pumping losses of the engine can be reduced and thereforexe2x80x94in comparison with conventional engine technology having port fuel injection and homogeneous stoichiometric operationxe2x80x94considerable fuel efficiency improvements can be obtained. Different construction principles which differ with regard to the system components are known for internal combustion engines of this type. The differences include, for example, the arrangement of the injectors and of the spark plugs, the number of inlet and outlet valves, the geometrical shape of the combustion chamber and of the piston, and the movement of air in the combustion chamber, which is brought about with the aid of the shape of the inlet port.
Depending on the arrangement of the components and on the method for forming the stratified fuel/air mixture, combustion systems are classified as wall-guided, air-guided or jet-guided systems. The systems which are currently under production are usually wall-guided systems having a centrally arranged spark plug and a laterally fixed injector. The fuel jet is conducted by the walls or a depression in the piston to the ignition point. This configuration suffers compromises with regard to the timing control of injection and ignition, thereby leading to losses in the thermal efficiency of the engine.
The same arrangement is used for air-guided systems, except that direct impact of the injected fuel jet onto the surface of the piston is prevented to the extent possible. For this purpose, the injected fuel jet is deflected by the movement of air in the cylinder toward the ignition point. However, systems of this type are difficult to optimize, since the air motion and turbulence level depend on the rotational speed of the engine.
Jet-guided systems constitute the most vigorous approach, in which the spark plug is arranged in the direct vicinity of the injector. A jet-guided system of this type is known, for example from U.S. Pat. No. 5,941,207. In this case, the stratified mixture reaches the ignition point by the injection jet being dispersed independently of the geometry of the combustion chamber and the movement of air. This makes possible the maximum thermal efficiency of an unthrottled, lean gasoline combustion process and also minimized exhaust-gas emissions. However, a fundamental problem of this approach resides in the durability of the fuel injector and of the spark plug due to the formation of deposits on the electrodes of the spark plug and due to coking of the injector because of insufficient cooling.
The internal combustion engine according to the invention having direct fuel injection comprises at least one cylinder having lateral cylinder walls and a cylinder head and a piston arranged in the cylinder, the piston and the cylinder together bounding a combustion chamber. The combustion chamber has at least one air inlet which is designed and is arranged on the cylinder in such a manner that when inlet air is admitted a preferably inclined swirling movement of the inlet air arises in the combustion chamber. A swirling movement is characterized by a vortex rotating about the cylinder axis or stroke axis of the piston.
Furthermore, in the internal combustion engine a fuel injector for injecting a fuel jet into an injection volume and an injection device are arranged centrally on the upper side of the combustion chamber, i.e., in the cylinder head, the electrodes of the injection device being situated outside the abovementioned injection volume of the fuel injector. The electrodes are not therefore directly affected by the injected fuel. The injection volume is defined as the region of space which is reached by the liquid particles of the injected fuel. The internal combustion engine is distinguished by the fact that the abovementioned injection volume is asymmetrical, the injection volume furthermore being shaped and aligned within the combustion chamber in such a manner that the electrodes of the spark plug are situated in a volume occupied by the fuel spray arising from evaporating fuel during and after the injection.
The internal combustion engine designed In the above-described manner has the advantage of a long service life since sooty carbon deposits on the spark plug and coking of the fuel injector are prevented or minimized because the spark plug is not directly affected by fuel droplets as the electrodes of the spark plug are situated outside the injection volume, i.e., the volume containing liquid fuel droplets. At the same time, however, a reliable and robust ignition of the air/fuel mixture is ensured as the electrodes are arranged in a region occupied by evaporated fuel.
The electrodes of the spark plug are preferably arranged in a region which is brushed over during rotation of the asymmetrical injection volume about the cylinder axis or stroke axis of the piston. Such a rotation of the injection volume is ensured by the swirling movement of the inlet air, which movement is produced by the design of the air inlet.
According to a development of the invention, the upper side of the combustion chamber, i.e., the cylinder head, is in the form of a pentroof. This shape of the cylinder head permits accommodation of the injector and the spark plug favorably.
Furthermore, the upper side of the piston can have a trough-shaped depression which is used in an advantageous manner to deflect the injected fuel and to distribute it in the combustion chamber.
According to a development of the invention, the at least one cylinder of the internal combustion engine has at least one inlet valve and at least one outlet valve, the abovementioned valves being actuated variably so that exhaust gases can be trapped in the cylinder, this is sometimes called internal exhaust-gas recirculation system. Variable valve timing (VVT) mechanisms, which permit adjustment of the opening and/or closing times of the valves are known in the art. In the ease of internal exhaust-gas recirculation, a late closing of the outlet valves ensures that part of the combusted exhaust gases flows back into the combustion chamber to produce more favorable conditions there for the following operating cycle.
The fuel injector is surrounded within the cylinder head on the upper side of the combustion chamber by a coolant jacket. Such a complete enclosure of the fuel injector with a coolant jacket is possible, in particular, by the asymmetrical shape of the injection volume, said shape, for its part, permitting an inclined installation of the fuel injector. This then produces the required space and distance between the injector and the spark plug for the coolant jacket. Cooling of the fuel injector advantageously prevents coking of the fuel injector.
The invention furthermore relates to a method for producing an air/fuel mixture in the combustion chamber of an internal combustion engine, a preferably inclined swirling movement being produced in the combustion chamber, and a fuel jet being injected into an injection volume in the combustion chamber. The method is distinguished by the injection volume being asymmetrical and the electrodes of a spark plug being situated outside the injection volume and inside a volume occupied by evaporated fuel. The electrodes are preferably situated in a region which the injection volume brushes over during rotation about the cylinder axis.
Advantageously, by this method, the electrodes are not directly affected by the liquid fuel in the fuel jet. Yet, ignition is ensured since by fuel evaporation and/or by the swirling movement of the inlet air about the cylinder axis, evaporated fuel enters into the ignition region, i.e., the location of the electrodes.
In the case of the abovementioned method, a stratified air/fuel mixture is produced in the combustion chamber. A stratified mixture of this type makes possible high, ignitable fuel concentrations in the region of the spark plug while the remaining volume of the combustion chamber is filled with a mixture of lower fuel concentration. This permits, in particular, efficient lean operation of the internal combustion engine.
According to a development of the method, during the induction stroke of the internal combustion engine a first quantity of fuel is injected into the combustion chamber to produce a homogeneous air/fuel mixture. During the subsequent compression stroke, a second quantity of fuel is injected to produce a stratified mixture. The division of the entire injected quantity of fuel into two parts provides an additional parameter for controlling engine operation which can be used to optimize the operation of the internal combustion engine.
Advantages provided by the present invention are an improved method for producing an air/fuel mixture. The engine and method ensure a longer service life and operability of the injector and spark plug.
Furthermore, the method is preferably implemented in such a manner that in all of the operating modes of the internal combustion engine the same flow ratios are produced in the combustion chamber. These flow conditions can be prescribed by structurally comparatively simple and unchangeable measures, so that overall a cost-effective and robust construction of the internal combustion engine can be obtained.